1. Field of the Invention
This invention is concerned with box beam boom arms for use as a boom assembly on loader vehicles. More particularly the invention is concerned with welded joints between boom arm side walls and boom arm top and bottom plates. The top and bottom plates are provided with edges formed to act as side wall locating guides, weld beds and weld blow through dams.
2. Description of the Prior Art
In boom assemblies of loader vehicles several types of constructions are used. One of the most common structures is seen in loader-backhoe vehicles that have boom arms fabricated from pairs of channel section members welded together in an edge-to-edge relationship to form a box structure of generally rectangular cross section. The channels used may also be tapered channels so that the load can be allocated toward a larger box section portion as necessary.
Another typical loader boom assembly uses a pair of massive plates of flat stock generally unified into an assembly through the use of cross ties or transverse torque tubes. Of course these plates do not require welding as they do not form box sections.
Box section boom arms constructed from a plurality of plates are also well known. Typically a pair of boom arms are utilized to make a boom assembly. Cross ties connect the inboard side walls or side plates of each arm together. Each arm is pivotally attached to the loader frame and necessary maneuvering cylinders are connected to the boom assembly at sundry locations.
The prior art boom arms have been welded using one of three typical fabrication methods. In a first technique the side plates, cut out in the proper shape, are provided with backup strips extending peripherally around the inboard sides of each of the side plates. These backup strips are set back from the outer edge of the plates a distance equivalent to the thickness of the flat plate stock top and bottom plates. Extensive forming of these relatively small backup strips, typically rectangular stock 25 mm.times.6 mm, is necessary to make them follow the curvalinear perimeter of the boom arm side wall plates. After the backup strips are bent to shape they are first tack welded in position and then continuously welded to the host side wall. The backup strips are not made long enough to preclude the necessity of joints between adjacent strips, hence secondary strips are welded to the backup strips to provide a continuous bead or barrier to prevent subsequent weld blow through. After the strips have been welded into position the top and bottom plates may be welded to the side walls to form the box boom arm. The top plate and the bottom plate have bevel cut edges such that the transverse dimension of the inboard side of the top plate is greater than the transverse dimension of the outboard side. The top is then welded to the pair of side plates with the weld pools filling the space defined by the straight walls of the side wall plates and the slanted walls (due to the beveled cut) of the top plate. The weld is narrow at the inner junction between the top plate and the two side walls and wider as the weld progresses outwardly. The backup strips are necessary to prevent weld blow through, but after the welding operation, are useless and unnecessary. They are not removed, however, due to the lack of accessibility. The bottom plate is welded in place in a similar manner.
The disadvantages of this first type of fabrication are:
1. The cost of positioning the blow through preventing backup strips;
2. The potential of weld blow through even with the backup strips due to slight spaces between the strips and the side wall surface or slight gaps at the butted ends of the backup strips; and
3. Distortion of the top and bottom plates as they are heated and cooled in the welding process due to differences in plate surface area due to the bevel cut edges. With the fabrication technique of this invention top and bottom plates will not have a tendency to deform as they are heated and cooled due to the slightly increased thickness of the top and the bottom plates.
Two other fabrication methods have been tried and utilized, but neither of them provide the cost and structural advantages of the instant invention. Each of the other techniques are variations of the technique described in detail above, however, in one embodiment the top plate outboard surface transverse dimension is equal to the overall width of the box boom arm structure and the inboard surface of the top plate has a transverse dimension equivalent to the wall-to-wall inside dimension of the box. The bottom plate is the same as the prior art bottom plate described above.
A third embodiment known in the prior art uses a top plate and a bottom plate having dimensional properties like the top plate as described in the second embodiment above.
All three of these embodiments are very costly due to the necessity of the backup strips to prevent weld blow through. Weld blow through may lead to poor weld penetration and a weak section in the boom assembly weldment.
The top plate and bottom plates of the instant invention are roll formed by passing a billet through a plurality of shaping and sizing dies that result in the special cross sectional shape necessary for the top plate and the bottom plate. The roll forming dies may be of a type that can be adjusted to provide slight changes in plate thickness or width as dictated by design requirements of different size boom arm assemblies. If significant dimensional changes are desired it may only be necessary to change forming dies at several of the forming stations rather than providing a whole new set of dies at a much greater cost.
Thus the primary object of this invention is to provide a box beam boom arm that is less costly to produce than state of the art box beam boom arms.
Another object of this invention is to provide a bo beam boom arm that can be welded together without the need for backup strips on the interior of the box beam.
Another object of this invention is to provide a top and bottom plate of a box beam that locates the side walls relative to each other preparatory to final welding.
Also an object of this invention is to provide a cross sectional shape for a top plate and a bottom plate that may be positioned adjacent the side walls of the structure such that different dimension weld beds are formed.
Advantageously, it has been found that several different cross sectional dimension top and bottom plates can be formed from a set of roller dies through substitution of the finishing dies of such set. Thus tooling costs for dimensionally different top and bottom plates are minimized.
Another advantage of this invention is that dishing and disfiguration of the top and bottom plates from welding and cooling operations is minimized due to the increased thickness of these plates over prior art plates coupled with good weld containment through the use of these top plate and bottom plates.